The present invention generally relates to tone control devices for thermal ink-transfer type printing apparatuses (hereinafter referred to as thermal printing apparatuses), and more particularly to a tone control device in a thermal printing apparatus, for controlling the density or tone of printing by controlling the size of printing dots according to the number of times a pulse current is applied to a thermal printing head.
Among terminal printers or hard-copy apparatuses such as wire-dot type, shuttle type, and ink-jet type printers, thermal printing devices are being developed as one of the more promising type. For example, this thermal printing device employs an ink film which is a polyester film having a thickness of 5 to 6 .mu.m coated with a kind of ink which melts due to heat on one surface thereof. The ink film is placed onto a recording sheet with the ink side making contact with the recording sheet, and a thermal printing head makes contact with a rear side of the ink film. When a current flows through the thermal printing head so as to generate heat at the printing head, the ink on the ink film melts at the position corresponding to the position of the printing head, and the melted ink is transferred onto the recording sheet. This thermal printing head comprises a plurality of head elements arranged in a row, and a current is successively applied to each of these head elements.
The density which determines the tone of the printed characters, diagrams, pictures, and the like, is determined by the area of each dot formed on the recording sheet due to the transfer of the melted ink onto the recording sheet. And, this area of the melted ink dot is determined according to the current applied to each of the head elements. Generally, the heat value becomes larger as the magnitude of the currents applied to the head elements become larger. As a result, the area of the melted ink dot becomes larger to increase the printing density, and the tone reaches near a saturated density. Accordingly, the magnitude of currents applied to the head elements were conventionally controlled in order to control the tone of printing. However, the currents applied to the head elements are generally large currents in the range of 5 to 20 Amperes. Thus, it was difficult to control such large currents with a quick response speed, and there was a disadvantage in that the size and cost of the control device became high. In addition, it was impossible to increase the response speed when controlling such large currents, and there was a disadvantage in that the printing speed could not be increased.